BACnet, Building Automation and Control Networks, ASHRAE 135.
BACnet is the open communication protocol for building automation and control networks, developed by ASHRAE and standardized as ANSI/ASHRAE Standard 135 and ISO 16484-5. BACnet defines an object model, analog input, analog output, binary input, binary output, schedule, calendar, trend log, notification class, and 30 plus other object types, a service model, read property, write property, subscribe COV, alarm, file transfer, and a network architecture supporting multiple physical layers, BACnet/IP, BACnet/SC, MS/TP, ARCNET. BACnet is the dominant BAS protocol in commercial buildings worldwide.
What BACnet, Building Automation and Control Networks, ASHRAE 135 means.
BACnet emerged in the 1990s as the ASHRAE response to vendor-proprietary BAS protocols that locked building owners into single-vendor systems. The protocol is intentionally vendor-neutral. Any BACnet-conformant device can communicate with any other, regardless of vendor. ASHRAE 135 has been updated continuously since the original 1995 release. The current edition is 2020, with addenda through 2024. The protocol supports a range of device complexity, from simple sensors with a few objects to full building controllers with thousands, multiple physical layers, and a standardized commissioning workflow. The international version, ISO 16484-5 is widely adopted in Europe, the Middle East, and Asia.
What is a BACnet object.
A BACnet object is an addressable container that represents a sensor, an actuator, a setpoint, a schedule, a trend log, an alarm, or other building-control entity. Each object has an object identifier, object type plus instance number and a set of properties, current value, units, description, present-value priority array, alarm thresholds, COV increment. Standard object types include analog input, AI. A sensor reading, analog output, AO. A setpoint or commanded value, binary input, BI. A status, binary output, BO. A command, multistate value, calendar, schedule, notification class, trend log, file, device. Each device exposes an inventory of objects. The BAS configuration tool reads and writes object properties to commission and operate the system.
What is the BACnet priority array.
Output objects, AO, BO, multistate output support a priority array with 16 priority levels. Each priority level can hold a commanded value. The actual output is the highest-priority non-null value. Priority 1 is manual life-safety, rare. Priority 8 is manual operator override. Priority 16 is the lowest, typically program control. The priority array allows multiple programs and operators to command an output without overwriting each other. Each writes to its own priority level, and the priority array resolves conflicts. Operator overrides at priority 8 win over program control at priority 16.
What is COV, Change of Value.
COV, Change of Value is the BACnet notification mechanism where a device sends an unsolicited update when an object property changes by more than a configured increment. COV reduces network traffic versus polling. The BAS supervisory tool subscribes to COV notifications from each sensor, and the sensor only sends an update when its value changes significantly. Each subscribing client controls the COV increment per subscription. COV notifications include the new value and an aging time so the supervisory tool can detect a sensor that has gone silent.
Common questions
What physical layers does BACnet support.
What is BTL.
Can BACnet integrate with other BAS protocols.
What is BACnet/SC.
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